💡 Detailed Explanation
The correct choice is Option 2: 'Because he overslept in the morning and missed the train.' The student explains: '実は昨日、朝起きるのが遅くなって、電車を逃しちゃったんだ' (Actually yesterday, my waking up in the morning got late, and I missed the train). When asked if he overslept ('寝坊したの？'), he confirms it was due to his alarm clock not ringing. Thus, Option 2 is the correct reason.